The use of broadband networks, such as asynchronous transfer mode (ATM) networks, for the transfer of multimedia information, such as video, voice and data, has become increasingly popular due to the high data rate and flexibility associated with such networks. ATM networks, for example, utilize a cell-oriented switching and multiplexing technique, and are based upon the use of short fixed-length "cells". ATM and other broadband networks have been embraced by the computer and telecommunication industries for networking existing and future multimedia applications, such as video conferencing, video-on-demand, and telephone applications.
An ATM network is a connection-oriented network, in which each transfer of information between a source device and a destination device is preceded by a call to a network connection manager to establish a connection, often referred to as a virtual channel, between the devices. Thus, the route between the devices is established prior to data transfer, and the connection is held during the entire transmission. The basic concept in establishing an end-to-end connection in an ATM network is the request for a series of virtual channel links from the source device to the destination device. The virtual channel (VC) on each link of the overall connection is identified in each transmitted cell by a virtual channel identifier (VCI) in the cell header, as well as by a virtual path identifier (VPI), which identifies the virtual path (VP) to which the virtual channel belongs. Thus, in order to route arriving cells, each switching node in an ATM network will typically include a routing translation table, which provides the VPI/VCI translation information for the switching node.
Currently, multimedia information may be transmitted across ATM and other networks to an end user by means of an integrated transport protocol, whereby the various monomedia components within the composite multimedia signal are encoded and transported together. This integrated transport approach, however, imposes several limitations, which, if overcome, could greatly expand the flexibility and quality of multimedia communications, particularly across heterogeneous networks. For example, it is not expected that all end-user devices and network service subscribers would desire, or be capable of accessing, all monomedia components in a multimedia presentation. For example, in a conference call, some end users may desire to access only voice and data, or just voice, and not video.
In addition, in the end-to-end path of multimedia sessions, heterogeneous network technologies, such as switches with different switch fabrics or diverse transport links, hereinafter referred to as "network segments," may be involved. For subscribers and locations where all switch fabrics and links are not available, only a subset of monomedia elements of a multimedia session may be supported. For example, if one or more network segments are wireless, the radio access (or the wireline portion of the wireless network) may only support voice, and not support data or video.
Furthermore, an integrated transport approach requires the same compression technique to be applied to each monomedia component of the composite multimedia signal. Thus, with an integrated transport approach, media specific compression techniques, which may be more efficient, cannot be directly exploited.
An important characteristic of a multimedia traffic session is the timing relationship between each monomedia component within the composite multimedia signal. If the inter-stream synchronization, or temporal correlation between monomedia streams, is not properly maintained throughout the transmission and processing of a multimedia application, the presentation will not be properly synchronized and will exhibit jitter. This problem arises due to the random delay in wide-area networks as monomedia information segments traverse through the network and interact with traffic from other sources.
As apparent from the above-described deficiencies with conventional systems for transporting multimedia information, a need exists for an improved method and apparatus for separately transporting each monomedia stream of a composite multimedia signal across a network, such as an ATM network. A further need exists for a multimedia communication system providing improved inter-stream synchronization between monomedia streams.